1. Field of the Invention
This invention relates to a tissue assessment apparatus for assessing the state of tissue such as bone using ultrasound, and in particular, to measurement of the speed of ultrasound propagated in tissue.
2. Description of the Related Art
In a tissue measurement apparatus using ultrasound, ultrasound is transmitted to a part of an organism (e.g. the heel), and the ultrasound which has passed through or been reflected by the organism is received. Based on a signal obtained from the received ultrasound, the speed of sound in the tissue or the degree of attenuation of the ultrasound by the tissue is computed as an indicator of the state of the tissue. An example of such an apparatus is a bone assessment apparatus used to examine for example the heel bone or calcaneous. In addition to tissue assessment apparatuses using only ultrasound, moreover, other apparatuses using both ultrasound and X rays have been proposed.
It is however known that when a tissue assessment is performed by means of ultrasound, it is impossible to obtain a precise measurement or assessment of speed or attenuation of ultrasound when an air layer is present in the path from the transmitting ultrasound transducer to the receiving ultrasound transducer. This is due to the fact that the ultrasound is reflected or attenuated by the air layer. In order to eliminate the air layer which interferes with ultrasonic examinations, therefore, the sample (tissue) and ultrasonic transducers were conventionally immersed in a measuring bath containing an acoustic matching material such as water, and predetermined measurements were carried out by transmitting and receiving the ultrasound through this material.
However, in the method where the sample was immersed in an acoustic matching material, the subject occasionally experienced discomfort, and it was difficult to ensure that the procedure of measurement was hygienic.
In Japanese Patent Application No. HEI-6-7010 (Japanese Patent Laid-Open No. HEI-7-204205), the Applicant proposes a tissue assessment apparatus which resolves these problems. In this apparatus, the front of the ultrasonic transducer is covered with a cover (or membrane) that can deform, the cover is filled with an acoustic matching material, and the cover is brought in contact with the sample. In other words, the sample is gripped between two ultrasonic transducers facing each other, and the propagation time of the ultrasound between the both transducers is measured. The propagation speed of the ultrasound in the sample, i.e. in the tissue, is then found by dividing the distance between the transducers by this propagation time. According to this method, the deformable cover easily comes into contact with the surface of the sample, so no air layer is present between the transducers and the sample.
In this art, however, a problem arises in that when the surrounding conditions, in particular the temperature or air pressure change during a measurement, the properties of the acoustic matching material also change and give rise to errors in measured values.
In this art, the distance used to compute speed of sound in the sample tissue comprises the thickness of the acoustic matching material, and the time used to compute speed of sound in the sample tissue comprises the time required for ultrasound to propagate through the acoustic matching material. Hence even for the same sample, if the properties of the matching material change due to a change in the measuring conditions, the value of speed of sound obtained may be different. Further, if there is an error in the speed of sound, there is a possibility of an error arising in other assessment values computed from the speed of sound.
When for example castor oil is used as the acoustic matching material and the temperature of the oil changes by 1.degree. C. in the vicinity of room temperature, the speed of sound in the oil changes by approx. 3 m/s. Therefore, when for example the thickness of castor oil is approx. 4 cm and the thickness of the sample is approx. 6 cm in the aforesaid apparatus, and the oil temperature changes by 1.degree. C., an error of about 1.2 m/s arises in the measured result for the speed of sound in the sample.
If an air cell becomes mixed with the acoustic matching material in the cover of the aforesaid apparatus, further problems arise. In this case, when the atmospheric pressure of the measuring environment changes, the air cell expands or contracts. The deformation of the cover pressing on the sample therefore changes due to the atmospheric pressure, and the average thickness of the acoustic matching material changes as a result. This leads to errors in the measured value of the speed of sound.
The thickness of the acoustic matching material also changes if the material leaks out of the cover due to long periods of use.
The part of the cover in contact with the tissue has excellent transparency to ultrasound and is formed of a thin film such as a polyurethane sheet which is highly flexible. When used for long periods however, the flexibility of the cover changes, so the degree to which the cover is deformed changes even when pressed against the sample by the same pressure. This also causes the thickness of the acoustic matching material to vary, and leads to errors in the measured result for the speed of sound.
In general, the speed of sound in the acoustic matching material and in the sample tissue are not the same, hence when the average thickness of the material changed due to a variation of environmental conditions, long periods of use or a change in the flexibility of the cover in the aforesaid technique, there was a risk that a different result would be obtained for the speed of sound even if the same sample were measured.